Bubble Waves – CwS Midterm

This experiment looked at the effects of audio waveforms on soap bubbles. We knew that the air being pushed by the speakers would effect the physical shape of the bubbles, but we wanted to see if there was something more to it besides dancing and jiggling colors in relation to audio frequency. Read more below.

First we fashioned a lightbox from IKEA LED lights, a plastic basket and some opaque plastic we got at a shop in Chinatown.

Our soap recipe called for children’s bubble solution, distilled water, dish detergent, and glycerin. Glycerin was the key ingredient for keeping the bubbles sturdy throughout the experiment.

The color of the bubble is related to the thickness of the soap film. When light waves hit a bubble, the bubble reflects the opposite, or complimentary, color of the waveform. Therefore if yellow light is hitting the bubble then that part of the bubble will reflect blue. Yet, there is more going on there than just a simple reflection of light waves.

The wall of a bubble is actually two surfaces, an inner and an outer layer. What we see when we are looking at the bands of color in a bubble is actually the combination of the two light waveforms which can have additive or deleterious effects on the resulting color.

The skin of a bubble glistens with the complementary colors produced by interference. If we were to look at a highly magnified portion of a soap bubble membrane, we would notice that light reflects off both the front (outside) and rear (inside) surfaces of the bubble, but the ray of light that reflects off the inside surface travels a longer distance than the ray which reflects from the outside surface. When the rays recombine they can get “out of step” with each other and interfere. Given a certain thickness of the bubble wall, a certain wavelength will be cancelled and its complementary color will be seen. Long wavelengths (red) need a thicker bubble wall to get out of step than short wavelengths (violet). When red is cancelled, it leaves a blue-green reflection. As the bubble thins, yellow is cancelled out, leaving blue; then green is cancelled, leaving magenta; and finally blue is cancelled, leaving yellow. Eventually the bubble becomes so thin that cancellation occurs for all wavelengths and the bubble appears black against a black background.

This surprising complete cancellation is due to the different way light reflects from the two surfaces. When light reflects from the outside surface of the bubble (an air-to-water surface) the direction of vibration of the wave is reversed – all “up” vibrations are turned into “down” vibrations and vice versa. (The same thing happens if you send a vibration along a rope tied to a wall; the reflected pulse is upside-down after reflection from the wall. ) When light reflects from the inside surface of the bubble (a water-to-air surface) the direction of the vibration is not changed. If the skin of the bubble is very thin, much shorter than the wavelength of visible light, then the two reflected rays of light will always meet crest-to-trough and destructively interfere. There will be no visible reflection, and the bubble looks black. When you see this happening at the top of a soap bubble you know the bubble is only about one millionth of an inch thick and will soon pop.

While it’s hard to tell in the video whether the changes in the bubble colors are due to sound frequency or just the pressure of the sound waves, we can still see the changing colors as the bubble gets closer to popping.